Describe smart comment support for Enum Types

[yasithA]

Description

Improved documentation on Enums by describing support for smart comments on PostgreSQL enum types.

Performance impact

N/A

Security impact

N/A

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[benjie]

Thanks!